Excited frame vibratory conveying devices are known in the art. U.S. Pat. No. 4,313,535, incorporated by reference herein, teaches a typical excited frame conveying apparatus. An apparatus such as this generally includes a vibratory drive mounted on an elongated frame and which rests on a floor or other supporting structure. Extending upwardly from the frame, and inclined in the direction of the infeed end of the conveyor are a plurality of leaf springs. An elongated conveyor bed is mounted on the distal ends of the leaf springs and is supported by them such that the bed is generally parallel to the frame and oriented in a substantially horizontal position.
Due to the resiliency of the leaf springs, the bed is capable of moving relative to the frame in response to a force applied to the bed by the vibratory drive. During operation of the conveyor, the vibratory drive produces an oscillating vibratory force. This vibratory force is usually generated by counter-rotating eccentric weights which are subcomponents of the vibratory drive. Because the vibratory drive is mounted on the frame, it imparts vibratory motion to the frame which is transferred through the leaf springs to the bed. Thus, the bed vibrates at substantially the same frequency as the drive and frame.
In operation, if the bed is displaced from its "at rest" position and then allowed to oscillate freely, it will oscillate at its natural or "harmonic" frequency. This natural frequency of the bed is dependent on both the combined spring constant of the springs supporting the bed as well as the mass of the bed. Generally, at frequencies near the natural frequency of the bed, the vibration of the vibratory drive is amplified significantly by the springs, resulting in substantially more movement of the bed than the frame. The maximum vibrational amplitude of the bed is attained when the frequency of the vibratory drive is the same as the natural frequency of the bed and springs.
In view of the design of the leaf springs, the bed supported by same is restricted to a given range of motion. In particular, the bed and frame remain substantially parallel to each other at all times. As the bed vibrates freely, it moves in a first direction that is generally upward and toward the outfeed end of the conveyor and then in a second, and opposite direction that is generally downward and toward the infeed end of the conveyor. This vibratory motion of the bed tends to "bounce" the product along the bed from the infeed end to the outfeed end.
As compared to other types of vibratory conveyors, less vibration is transferred to the floor or other supporting structure by an excited frame design because of its relatively light weight and small vibration amplitude of the frame compared to the vibration amplitude of the bed. The low level of vibration transferred to the surrounding structure is a chief advantage of the excited frame vibratory conveyor.
While vibratory conveyors have operated with varying degrees of success in handling various products, there have been shortcomings which have detracted from their usefulness. For example, if the vibrational amplitude of the bed is allowed to become too great, the leaf springs supporting same become repeatedly over-stressed. This results in premature wear and sometimes failure of the springs. This problem stems, in part, from the nature of the prior art excited frame design, inasmuch as the bed is allowed to vibrate freely in response to changing conditions which affect the vibrational amplitude of the bed.
In addition to the foregoing, commercial operators of vibratory conveyors want the conveyors to be as versatile as possible. For example, the operators want the conveyor to be able to handle a wide array of products at different conveying speeds. Thus, an operator of a vibratory conveyor often wants to control both the frequency of vibration and the amplitude of vibration of the bed in order to control the conveying speed of the product. Also, the operator may wish to adjust both the frequency and amplitude to avoid damage to more fragile or delicate products.
Generally then, it is desirable for the frequency of vibration to be directly correlated to the amplitude of vibration since both contribute to the conveying speed. In other words, it is usually desirable for the vibration amplitude to increase as the vibration frequency increases and vice versa, since both the amplitude and frequency are generally directly related to conveying speed. If the vibratory drive is operated at frequencies that are below the natural frequency of the bed, an increase in the frequency of the vibratory drive will bring the frequency of the vibratory drive closer to the natural frequency of the bed, thus increasing the vibrational amplitude of the bed. Therefore, the vibratory drive is usually operated at a frequency that is less than the natural frequency of the system to ensure a direct correlation of the frequency and amplitude of the bed.
As a general matter the natural frequency of the bed is inversely proportional to the mass of the bed. In other words, the heavier the bed, the lower its natural frequency. As a practical matter, the mass of the bed also includes the product being supported by the bed. Thus, as the mass of the product on the bed increases, the natural frequency of the bed will decrease. As mentioned above, the vibratory drive is generally operated at a frequency that is lower than the natural frequency of the bed. Consequently, as the mass of the product on the bed increases and the frequency of the vibratory drive remains constant, the natural frequency of the bed will decrease and approach the frequency of the vibratory drive, which in turn, results in an increase in the vibrational amplitude of the bed. As will be readily recognized the mass of beds in most commercial environments will be changing dynamically over time in view of day to day manufacturing contingencies.
As should be understood, the degree of amplification of the vibration produced by the vibratory drive increases exponentially as the frequency of the vibration approaches the natural frequency of the bed. Generally, the degree of amplification is limited only by any frictional damping forces present in the system, which in a conveyor of this type is generally negligible. Thus, as the frequency of the drive and the natural frequency of the bed approach one another, the amplitude of the bed increases to a degree which encourages the premature failure of the springs.
As noted above, in a commercial environment, the mass of the product on the conveyor bed can change dynamically over time. For example, if a downstream stoppage occurs, product may back up along the production line and become more densely packed on the conveyor bed. Also, certain types of product may contain substances which adhere to the conveyor bed, causing a buildup of the same substance on the bed, resulting in an increased mass of the bed. Still further, a product may be processed which has a higher density than that for which the conveyor was originally designed.
In addition to the foregoing, while the vibrational amplitude of the bed will generally increase in response to an increase in the mass of the bed, the amplitude will also generally increase in response to an increase in the frequency of the vibratory drive. For example, the operator of the conveyor may increase the vibrational frequency of the vibratory drive in an effort to increase the conveyed speed of the product. Varying the frequency of the vibratory drive is accomplished by varying the speed of the motor which drives the eccentric weights, such as by using a variable speed drive unit in conjunction with an a/c motor. An increase in the speed of the motor will increase the frequency of the vibratory drive, bring the bed closer to its natural frequency, resulting in an increase in the vibrational amplitude of the bed.
In both cases noted above, whether by an increase in the vibrational frequency of the drive or, by an increase in the mass of the bed, the vibrational amplitude of the bed may increase to unsatisfactorily high levels, thereby encouraging the premature failure of the associated springs and possibly damage to other parts of the conveyor or adjacent processing equipment. Thus, an excited frame conveyor must normally be monitored closely by an operator so that the frequency of the drive remains sufficiently below the natural frequency of the bed in order to prevent premature failure of the springs. This can sometimes be difficult if not impossible to accomplish given the changing nature of the bed mass in response to changing product manufacturing and processing conditions.
To address these perceived disadvantages, users of such devices have considered lowering the drive frequency to compensate for increased bed mass. However this has not been satisfactory because this will generally result in an undesirably slow conveying speed. Yet further, the installation of enhanced springs will not solve the problem, but will simply increase the natural frequency of the bed, which inturn, will require a correspondingly higher operating frequency of the vibratory drive in order for the excited frame conveyor to function properly.
Therefore it has long been known that it would be desirable to provide a vibratory conveyor which achieves the benefits to be derived from similar prior art devices and assemblies, but which avoids the shortcomings and detriments individually associated therewith.